maint: replace llvm-tools-preview with llvm-tools

This commit replaces all occurrences of llvm-tools-preview with llvm-tools, because it became a stable feature with Rust 1.60, thereby making the -preview prefix obsolete. Beside this, llvm-tools-preview is a redirection to the latter one anyway.

However, it does not work the other way around. At the moment, it is not possible to execute the maint/coverage script on a NixOS system, because the rustup component list only includes llvm-tools, but not llvm-tools-preview, thereby making this change necessary.